The present invention relates to a display system for producing color images by light-modulating (spatially light-modulating) the lights of different color elements obtained through sequential color separation and, more particularly, to a display system which adopts a pulse-width modulation for light modulation.
A pulse-width modulating display system is generally provided with a spatial light modulator that generates image lights by partially reflecting or transmitting light from a light source, and sequentially projects the image lights produced by the modulator on a screen to display images. The spatial light modulator generates the image lights on the basis of input image signals from external devices such as PCs and video equipment. The contrast of images displayed on the screen is normally defined by modulating the pulse-width of signals that execute ON/OFF control of the modulator.
There is described an example of the pulse-width modulating display system in Japanese Patent Application laid open No. HEI10-78550. The conventional display system obtains color images by light-modulating the lights of different color elements which have undergone sequential color separation.
FIG. 1 is a block diagram showing the configuration of the conventional display system. With reference to FIG. 1, the display system comprises a light source 1, a color switch filter unit 31 used in the sequential color separation of white light from the light source, a spatial light modulator 2 for sequentially receiving the lights of different color elements obtained by the sequential color separation and generating the image lights of the color elements by partially reflecting the input lights in a prescribed direction, a projection lens 3 for projecting the image lights of the color elements sequentially generated by the spatial light modulator 2 on a screen 4, and a drive circuit 51 for driving the spatial light modulator 2 and color switch filter unit 31 in synchronism based on an image signal 101 and a frame synchronous signal 102 sent from the outside (e.g. PC).
Examples of the spatial light modulator 2 include a digital micromirror device (DMD) comprised of hundreds of thousand of micro-mirrors whose angles of gradient are adjustable. The micro-mirrors of DMD correspond to, one each, the picture elements (pixels) of images displayed on the screen 4. Any images can be presented by controlling the angle of each micro-mirror. To be specific, each of the micro-mirrors is selectively adjusted at the first angle for reflecting the light in a direction to avert it from the projection lens 3 or the second angle for reflecting the light to the projection lens 3, thus creating desired images on the screen 4. The angle control of the micro-mirrors is executed based on a modulation signal 103 fed from the drive circuit 51.
The color switch filter unit 31 includes a color wheel 41 provided with plural color filters (Red, Green, Blue) which have different spectral transmittance characteristics and are arranged one by one in the circumferential direction, a motor 11 which supports the center of the color wheel 41 for rotating the wheel 41 in a prescribed direction, a couple of elements (light emitting element 12 and light acceptance element 13) disposed opposite to each other with the color wheel 41 between them, and a color wheel control circuit 81 for controlling the rotation of the motor 11.
The color wheel control circuit 81 receives a color wheel phase signal 112 from the light acceptance element 13 as well as a color switch control signal 104 from the drive circuit 51, and sends a motor control signal 111 to the motor 11. The color wheel phase signal 112 includes information on the rotation cycle of the color wheel 41. The information is derived from the timing of the reception of light that the light emitting element 12 emits through a hole made in a prescribed position on the color wheel 41 to the light acceptance element 13.
The image signal (video signal) 101 fed from the outside consists of image signals relative to respective colors Red, Green and Blue (including intensity information), which are sequentially output with respect to each frame. Besides, frame synchronization is performed based on the frame synchronous signal 102. The modulation signal 103 is a signal for controlling the angles of the micro-mirrors of the spatial light modulator 2 according to the image signal 101 (image signals for colors R, G and B). That is, the respective micro-mirrors are set at the first angle or the second angle based on the modulation signal 103. The color switch control signal 104 controls color filter (R, G, B) switching to execute the color separation by the color wheel 41. The color filters can be switched in timing with the changeover of the image signal 101 based on the frame synchronous signal 102. The color filter switching is carried out by rotating the color wheel 41 in a light path.
In the following, the image display operation of the above-mentioned conventional display system will be described by taking the case where the image signal (R, G, B) 101 is fed into the drive circuit 51 for example.
The light radiated from the light source 1 enters in the color switch filter unit 31. When the image signal (Red) 101 is input on this occasion, the drive circuit 51 controls the angles of the micro-mirrors corresponding to the respective pixels in response to the signal (R) 101. Concretely, the drive circuit 51 feeds the modulation signal 103 with the spatial light modulator 2 to switch the angles of the respective micro-mirrors to the first angle (at which the micro-mirror reflects light to divert it from the projection lens 3) or the second angle (at which the micro-mirror reflects light toward the projection lens 3).
In addition, the drive circuit 51 switches the color filters of the color wheel 41 in synchronism with the angle control of the micro-mirrors (spatial light modulation in the spatial light modulator 2) according to the frame synchronous signal 102. More specifically, the drive circuit 51 feeds the color wheel control circuit 81 with the color switch control signal 104 to switch the filter set in the light path to the color filter (R) when the image signal (R) 101 is input. The color wheel control circuit 81 sends the motor control signal 111 to the motor 11 based on the color switch control signal 104 and the color wheel phase signal 112 received from the light acceptance element 13. In response to the motor control signal 111, the motor 11 rotates the color wheel 41 so that the light radiated from the light source 1 enters the color filter (R).
The light having entered the color filter (R) transmits therethrough and becomes light (R). The light (R) then enters into the spatial light modulator 2. The light modulator 2 spatially light-modulates the light (R) to generate image light (R). The image light (R) is projected on the screen 4 by the projection lens 3.
Subsequently, the image signal (Green) 101 is input. With this the drive circuit 51 feeds the spatial light modulator 2 with the modulation signal 103 to execute the angle control of the micro-mirrors according to the image signal (G) 101. At the same time, the drive circuit 51 feeds the color wheel control circuit 81 with the color switch control signal 104 to switch the filter of the color wheel 41 to the color filter (G). The color wheel control circuit 81 sends the motor control signal 111 to the motor 11 based on the color switch control signal 104 and the color wheel phase signal 112 received from the light acceptance element 13. In response to the motor control signal 111, the motor 11 rotates the color wheel 41 so that the light radiated from the light source 1 enters the color filter (G).
The light having entered the color filter (G) transmits therethrough to become light (G). The light (G) then enters into the spatial light modulator 2. The spatial light modulator 2 spatially light-modulates the light (G) to generate image light (G). The image light (G) is projected on the screen 4 by the projection lens 3.
After that, the image signal (Blue) 101 is input. Accordingly, the drive circuit 51 feeds the spatial light modulator 2 with the modulation signal 103 to execute the angle control of the micro-mirrors in conformity with the image signal (B) 101. At the same time, the drive circuit 51 feeds the color wheel control circuit 81 with the color switch control signal 104 to switch the filter of the color wheel 41 to the color filter (B). The color wheel control circuit 81 sends the motor control signal 111 to the motor 11 based on the color switch control signal 104 and the color wheel phase signal 112 received from the light acceptance element 13. In response to the motor control signal 111, the motor 11 rotates the color wheel 41 so that the light radiated from the light source 1 enters the color filter (B).
The light having entered the color filter (B) transmits therethrough to become light (B). The light (B) then enters into the spatial light modulator 2. The spatial light modulator 2 spatially light-modulates the light (B) to generate image light (B). The image light (B) is projected on the screen 4 by the projection lens 3.
As a result of these operations, the image lights R, G and B are sequentially projected to an enlarged scale on the screen 4. The switch in the image lights (R, G, B) is inappreciable to the human eye. Consequently, images in the respective colors shown by the image lights (R, G, B) are temporally superimposed, and thus recognized as a color image in human perception. The contrast of color images displayed on the screen 4 can be arbitrarily adjusted by modulating the pulse-width of the modulation signal 103 that controls the operation of the spatial light modulator 2.
In the following, a description will be given of the concrete configuration of the drive circuit 51 and color wheel control circuit 81 of the conventional display system.
FIG. 2 is a block diagram showing an example of the configuration of the color wheel control circuit 81. The color wheel control circuit 81 receives the color switch control signal 104 from the drive circuit 51 and the color wheel phase signal 112 from the light acceptance element 13 as its input signals. The color wheel control circuit 81 includes a frequency phase comparator 83 for comparing the frequency phases of the input signals to output an error signal, and an amplifier 84 for amplifying the error signal output from the frequency phase comparator 83 to output the amplified signal as the motor control signal 111.
FIG. 3 is a block diagram showing an example of the configuration of the drive circuit 51. The drive circuit 51 has a pulse-width modulating circuit and a color switch control circuit 68. The pulse-width modulating circuit is composed of a memory circuit 61, a write control circuit 63, read control circuit 64 and a color switch timing information table 65.
The color switch timing information table 65 includes the information indicating the timing for making a switch in color filters of the color wheel 41. The color switch control circuit 68 outputs the color switch control signal 104 in synchronism with the frame synchronous signal 102 for switching colors in the color switch filter unit 31 shown in FIG. 1.
The image signal 101 input from the outside is once written into the memory circuit 61. Necessary data is read out of the memory circuit 61, and sent to the spatial light modulator 2 as the modulation signal 103. The write control circuit 63 controls the operation of writing the image signal 101 into the memory circuit 61. The timing of the writing operation is determined based on the frame synchronous signal 102. The read control circuit 64 controls the operation of reading necessary data out of the memory circuit 61. The timing of the reading operation is determined based on the color switch timing in the color wheel 41 derived from the frame synchronous signal 102 and the color switch timing information table 65.
In the above-described conventional display system, the spatial light modulator 2 controls the switch between ON/OFF states on a pixel-by-pixel basis according to the modulation signal 103. The light is conducted to the projection lens 3 in the ON state, whereas the light is not conducted thereto in the OFF state. The produced image becomes brighter as the ON state continues longer. In the pulse-width modulation, the image signal indicates the contrast by taking advantage of this behavior. Examples of the spatial light modulator 2 capable of such pulse-width modulation include surface stabilized ferroelectric liquid crystal displays, etc. in addition to DMD.
The pulse-width modulation will be more fully described below.
It is assumed by way of example that 8 bits are used to define the contrast of respective R, G and B colors in the input image signal 101. FIG. 4 is a schematic diagram showing the per frame data structure of the image signal 101 for explaining the pulse-width modulation in the conventional display system.
One frame of the image signal 101 is time-divided into sections for data R, G and B. The respective data R, G and B is composed of time slots 1 to 255. Time slot 255 is allocated to bit 0, and correspondingly, other time slots are allocated to the respective bits as follows: time slots 254 and 253 for bit 1, time slots 252 to 249 for bit 2, time slots 248 to 241 for bit 3, time slots 240 to 225 for bit 4, time slots 224 to 193 for bit 5, time slots 192 to 129 for bit 6, and time slots 128 to 1 for bit 7.
Time (time slots) is allocated to the respective bit 7 (most significant bit: MSB), bit 6, . . . , bit 1, bit 0 (least significant bit: LSB) in a ratio of 27:26: . . . :21:20. Provided that time T is allocated to bit 0, times 128T, 64T, . . . , 4T, 2T are allocated for bit 7, bit 6, . . . , bit 2, bit 1, respectively. Besides, when the input image signal 101 indicates the contrast levels of 255, 254, . . . , 2, 1, and 0, the periods of ON state corresponding to the respective contrast levels are 255T, 254T, . . . , 2T, T, and 0.
Bit assignment information indicates the above-mentioned time allocation for each bit, and is normally prepared beforehand in the form of a bit assignment information table. The ON/OFF control of the spatial light modulator 2 is carried out according to the bit assignment information table. For example, in the case of representing a contrast level of 130 for a pixel, the spatial light modulator 2 controls the light in regard to the pixel so that the light reaches onto the screen during the time-domains of the bit 7 (128 unit time) and bit 1 (2 unit time), but does not reach thereto in the time-domains of other bits 6 to 2.
However, the conventional display system has the following problems.
Let it be assumed that, in cases, as for example, where the sequential color separation of white light is performed with the use of a color wheel having filters in three colors R, G and B, the color wheel rotates at a speed of 60 Hz and 8 bits are used to define the contrast of the respective colors. In this case, the minimum switching time in the pulse-width modulation is below 22 microseconds. It is necessary to use a spatial light modulator having response speed faster than the minimum switching time. In addition, the minimum switching time becomes even shorter when increasing the rotational speed or number of colors of the color wheel, or raising the level of contrast. As a result, higher response speed is required of the spatial light modulator. Nevertheless, there is a limit to the response speed of the modulator. Consequently, the minimum switching time in the pulse-width modulation has been limited due to the response speed of the modulator, and it has been difficult to increase the number of bits beyond a certain level. Incidentally, when the setting of the minimum switching time exceeds the response speed of the modulator, the brightness of images displayed by LSB is lowered, resulting in a deterioration of image quality in areas of low brightness.
Besides, provided that the spatial light modulator supports 1024xc3x97768 pixels, the maximum rate of pulse-width modulation signal transfer to the modulator is very high, up to 564 MHz per 64-bit width (=60 Hz*3*255*1024*768/64). Since higher signal frequency produces a higher level of noise, peripheral circuits become susceptible to malfunction, and further, power consumption increases.
Consequently, there have been made studies on a display system with low rate of pulse-width modulation signal transfer, in which a spatial light modulator having low response speed is usable while maintaining system""s performance. In the following, a description will be given of the display system described in Japanese Patent Application laid open No. HEI9-149350 as an example.
FIG. 5 is a schematic diagram of a color wheel used in the above-mentioned system. The color wheel is provided with a plurality of color filters (R, G, B) having different spectral transmittance characteristics, which are arranged one by one in the circumferential direction at a prescribed rate (spacing angle: 120xc2x0). The filter B is provided with a low-density segment 34 or a density (intensity) filter B+NDF in a certain angular range from the interface with the filter G. White light transmittance is decreased in the low-density segment 34 as compared to the other areas 32 of the filter B. The filters R and G are likewise provided with low-density segments R+NDF and G+NDF, respectively.
In the system, white light from the light source is sequentially separated into color lights R, G and B by rotating the color wheel in a light path. After that, the color lights illuminate DMD being the spatial light modulator, and image lights R, G and B from the DMD are sequentially superimposed on the screen. Thus, color images are produced in the same manner as the system depicted in FIG. 1. Since the respective color filters R, G and B of the color wheel are provided with the low-density segments, the intensity of the color lights R, G and B drops in the segments. Consequently, when the ON/OFF switch of light in the time-domains of low bits of an image signal, namely, bit 1 (2 unit time) is carried out for the lights having passed through the low-density segments, it becomes possible to extend the time for the low bits. The response speed of the DMD is regulated by the length of time of the low bits. Therefore, it also becomes possible to reduce the response speed according to the extension of the time for the low bits
However, there are following problems in the above-described conventional display systems.
In the display system depicted in FIG. 1, there is a limit to the rate of pulse-width modulation signal transfer as well as to the response speed of the available spatial light modulator, which necessarily causes a disadvantage in design.
On the other hand, in the display system having the color wheel of FIG. 5, the problem of the limit is resolved. However, there are produced new problems as follows.
It is preferable to use the intensity filter also for intermediate bits between high bits and low bits in order to smoothly present the contrast. In other words, it is necessary to provide the respective color filters with plural intensity filters of different degrees of intensity to achieve a good contrast presentation. The color wheel of the system has only one intensity filter with respect to each color filter, and it is difficult to achieve a good contrast presentation.
It is possible, but costly, to provide the respective color filters with plural intensity filters of different levels of intensity because the processes for manufacturing the color wheel are increased. This problem will be explained below.
The color wheel is generally produced by affixing color filters onto a transparent disk, or by depositing filter material on the surface of a transparent disk in a vacuum chamber. In the color wheel shown in FIG. 5, the respective three color filters are provided with two segments of different density levels, and 3xc3x972=6 filters are needed. Consequently, it is required to repeat the affixing process or depositing process six times. When forming three segments of different density levels in the respective color filters, 3xc3x973=9 filters are needed, thereby requiring nine times of the affixing process or depositing process. As is described above, when producing the color wheel with a disk, the manufacturing process is necessarily repeated {(the number of color filters)xc3x97(the number of segments formed for each filter)} times. Thus, setting of plural intensity filters of different density levels in the color wheel increases manufacturing processes and costs.
In addition, when setting plural patterns of intensity to be switched, it is required to produce the color wheel having filters corresponding to the intensity patterns if color separation and intensity switching are performed by using the same color wheel. This enormously raises the cost of production.
It is therefore an object of the present invention to provide a display system capable of raising limits on the response speed of the spatial light modulator and the rate of pulse-width modulation signal transfer at a low cost.
In accordance with an aspect of the present invention, to achieve the above objects, there is provided a display system comprising: a color switch means for switching lights of plural color elements to provide the respective lights of color elements one by one in sequence; a spatial light modulation means which is illuminated by the lights of plural color elements from the color switch means and generates image lights of the respective color elements; and an intensity switch means for switching two or more intensity levels of the respective lights of plural color elements or the respective image lights of the color elements, being separate from the color switch means.
In the display system of the present invention, the spatial light modulator generates the image lights based on an image signal input from the outside. The intensity switch means switches the intensity level of the respective lights. Accordingly, it becomes possible to allocate the high bits of the image signal for a pulse-width modulation signal in timing with brighter intensity filters, and to allocate the low bits thereof for the pulse-width modulation signal in timing with darker intensity filters. As a result, the minimum switching time of pulse-width modulation can be prolonged, thus enabling a reduction in the rate of pulse-width modulation signal transfer as well as the use of a spatial light modulator with low response speed.
Besides, the color switch means and the intensity switch means are individually provided to the display system, and therefore, color switching performed by the color switch means and intensity switching performed by the intensity switch means are controlled separately. That is, the color switching is carried out by using a color wheel. The intensity switching is carried out by using an intensity wheel being independent of the color wheel. Consequently, according to the present invention, it is possible to use the intensity wheel with plural different intensity levels for one color wheel. Thus, when setting plural patterns or levels of intensity to be switched, it is only required to produce the intensity wheel having filters corresponding to the intensity patterns, thereby dispensing with the need to produce the color wheel for each intensity pattern.
In addition, the separate color wheel and intensity wheel lead to a reduction in manufacturing process. In the case of, for example, providing the respective three color filters with segments of three different density levels, 3xc3x973=9 filters are needed for the color wheel shown in FIG. 5 as described above. Consequently, it is required to repeat the affixing process or depositing process nine times. On the other hand, in the display system according to the present invention, three affixing processes or depositing processes are required for manufacturing the respective color wheel and intensity wheel. That is, the affixing process or depositing process is repeated six times, lessened by three times as compared to the conventional display system. The gap in the number of manufacturing processes widens as the segments of different density levels increase.
Incidentally, the intensity switching may be carried out by a liquid crystal panel, by changing the brightness of a light source, or by changing the aperture of a projection lens. Besides, a light source emitting lights in plural different colors may be adopted as the color switch means. In this case, it is possible to dispense with the color wheel and intensity wheel, and there is no need to perform complicated synchronous control.